1. Field of the Invention
The present invention relates to a device for retaining a workpiece to be polished, the surface of which needs to be polished to be precisely and uniformly flat, such as a semiconductor wafer, an interlevel layer and a metal wire used in a semiconductor device, a memory disc, and a glass plate used in an LCD; and a method for producing the same.
2. Description of the Related Art
Conventionally, a semiconductor wafer, an interlevel layer and a metal wire used in a semiconductor device, a memory disc, a glass plate used in an LCD, and the like are polished to be precisely and uniformly flat in the following manner.
A workpiece to be polished as mentioned above is secured to a rotatable retaining plate in one of the conventional methods (1) through (3) described below. An artificial leather-like polishing cloth is attached to a rotating board located opposed to the retaining plate. A polishing liquid containing polishing grains is supplied between the polishing cloth and the workpiece while the polishing cloth and the workpiece are caused to slide with respect to each other.
(1) Wax mounting method: A metal or ceramic retaining plate is coated with wax melted by heat and then cooled down to a softening point of the wax, thereby securing the workpiece to the retaining plate.
(2) Vacuum chuck method: Suction openings having a diameter of 0.5 mm to several millimeters are formed in a metal or ceramic retaining plate by machining, and the workpiece is attracted to the retaining plate by a vacuum pump which absorbs air through the suction openings from near the rear and side surfaces of the retaining plate. Thus, the workpiece is secured to the retaining plate.
(3) No-wax mounting method: A retaining plate is provided with an artificial leather-like polymer foam material. The polymer foam material is moistened to obtain water in pores thereof. When water comes out of the pores by a pressure applied to the material through the workpiece, a negative pressure is generated in the pores. By the combination of the negative pressure and the surface tension of the water, the workpiece is secured to the retaining plate.
The above-mentioned three methods each have the following problems associated therewith.
(1) Wax mounting method
(a) After the workpiece is polished, the wax used for securing the workpiece remains on the rear surface of the retaining plate, which must be removed by troublesome washing steps.
(b) The workpiece cannot be heated to a temperature higher than the softening point of the wax.
(c) Gel or foreign substances such as dust, if contained in the wax, are transferred to the surface of the workpiece, thereby forming dimples in the workpiece during the surface polishing operation, which lowers the quality of the polished workpiece.
(2) Vacuum chuck method
(a) The workpiece is attracted and thus partially deflected toward the suction openings while the workpiece is being polished. When the polishing is finished, the workpiece recovers to its original state, and as a result the area which is recovered from the deflection state is not satisfactorily polished.
(b) The polishing liquid is pulled into the tube of the vacuum pump between the workpiece and the retaining plate and then through the suction opening, which causes malfunctions in and corrosion of the vacuum pump.
(3) No-wax mounting method
Due to the flexibility of the polymer foam material, it is difficult to transfer the flatness of the rigid metal or ceramic retaining plate to the surface of the workpiece by the principle of so-called profiling machining.
Further, all the above-mentioned conventional methods have the problem in that, while the workpiece slides against the polishing cloth, stress is concentrated along an outer periphery of the workpiece, causing an overhanging lip along the outer peripheral edge thereof to spoil the flatness.